The present invention relates to a system for generating a clock signal used when recording data on an optical disc, and more particularly to a generating system wherein the clock signal is extracted from wobble signal which is generated when a wobbled recording groove tracks formed on the disc is read out.
There has been known a Compact Disc-Recordable (CD-R) disc which is a write-once optical disc. The CD-R disc has a plurality of recording groove tracks in which data are recorded. The groove tracks are slightly wobbled in the radial direction of the disc by a wobble signal which is modulated with a frequency modulation. Therefore, when the CD-R disc is rotated, there is generated the wobble signal having a predetermined frequency (22.05 KHz). The carrier wave carries prerecorded information such as position information of the tracks.
In order to generate the wobble signal, a light beam is radiated on the groove tracks. The reflected beam is applied to a photodetector divided into two portions by a line optically parallel with a tangent of the groove tracks. A difference signal dependent on the difference between the outputs of the two portions is then applied to a band pass filter (BPF) having a transmitting range, the intermediate frequency of which corresponds to the predetermined frequency, that is 22.05 KHz. The rotation of the disc is controlled so that the average frequency of the extracted wobble signal becomes 22.05 KHz.
The extracted wobble signal is further used as a reference signal for producing a clock signal used at the recording. Namely, the wobble signal is a continuous signal having a frequency component which is synchronous with the rotation of the disc. When the phase of the clock signal is synchronous with that of the continuous signal, the clock signal accurately synchronizes with the rotation of the disc.
Meanwhile, the research and development of a write-once Digital Versatile Disc-Recordable (DVD-R) are much in progress. The recording capacity of the DVD disc is increased about 7 times as much as that of the CD-R disc. In order to increase the recording density, the distance between tracks of the DVD-R disc is decreased to substantially half of that of the CD-R disc. As a result, a part of the light beam applied on one groove track radiates the adjacent groove track, thereby causing a so-called crosstalk which cannot be ignored. When there occurs the crosstalk caused by the adjacent right and left groove tracks, the extracted wobble signal is interfered by the wobble component of the adjacent groove tracks so that the amplitude and the phase thereof deviate. In particular, when the phase is deviated, jitters inevitably occur, so that a clock signal which is accurately synchronous with the rotation of the disc cannot be generated.
More particularly, the crosstalk causes a deviation in a disc rotation control signal because of a phase difference between the reference clock signal and the extracted wobble signal. Accordingly, there occurs a difference in the period of time necessary for the rotation controlling operation dependent on the deviated rotation control signal to be effected, and a period of time it takes a phase-locked loop to generate the clock signal. Due to the inertia of the disc, it usually takes much longer time for the rotation control system to respond than the phase-locked loop. As a result, the phase of the clock signal is deviated from the phase of the rotation of the disc. Such a deviation causes the recording pits to be inaccurately formed in the groove track at recording as regards to the length thereof. Therefore, error occurs at reproduction of the disc.
An object of the present invention is to provide a clock signal generating system wherein the generated clock signal used at the recording of information is accurately synchronized with the rotation of the disc even if there occurs a crosstalk between adjacent groove tracks.
According to the present invention, there is provided a clock signal generating system for recording information on a disc which has a plurality of recording tracks wobbled by a wobble signal having a predetermined frequency and a plurality of prepits disposed at positions having a predetermined phase relation with respect to a phase of the wobble signal.
The system comprises generator means for generating a clock signal, extractor means for extracting the wobble signal, detector means for detecting the prepit and for producing a prepit signal, phase adjusting signal generator means for comparing the phase of the wobble signal with a phase of the prepit signal and for generating a phase adjusting signal, adjusting means for adjusting a phase of the clock signal based on the phase adjusting signal, and recording clock signal generator for generating a recording clock signal by an adjusted clock signal adjusted by the adjusting means.
The adjusting means comprises a phase shifter for shifting the phase of the clock signal.
The generator means comprises a PLL circuit for generating a clock signal in synchronism with the phase of the wobble signal.
In another aspect of the present invention, the generator means comprises a phase comparator for producing a phase difference signal representing a phase difference between the clock signal and the wobble signal and a PLL circuit having an oscillator for generating a clock signal based on the phase difference signal, the adjusting means comprises an adder disposed between phase difference comparing means and the oscillator in the PLL circuit, for adding the phase difference signal and the phase adjusting signal.
In a further aspect, the generator means comprises first phase comparator means for comparing a phase of the clock signal with a phase of reference clock signal and for producing a first phase difference signal, and a PLL circuit having an oscillator for generating the clock signal based on the first phase difference signal, the adjusting means comprises a phase shifter for shifting the phase of an extracted wobble signal, second phase comparator means for comparing a phase of the extracted wobble signal with a phase of the reference clock signal and for producing a second phase difference signal, and an adder disposed between the first phase comparator means and the oscillator in the PLL circuit, for adding the first phase difference signal and the second phase difference signal.
The generator means may have a voltage controlled oscillator for changing a frequency of the clock signal in accordance with a control voltage signal, the adjusting means may have a phase shifter for shifting a phase of an extracted wobble signal in accordance with the phase difference signal, control voltage generator means for generating the control voltage signal based on a phase difference between an output signal from the phase shifter and the clock signal.
In a further aspect, the generator means comprises a frequency divider for dividing a frequency of the clock signal and for generating a wobble signal frequency signal, first phase comparator means for comparing a phase of the wobble signal frequency signal with a phase of an extracted wobble signal and for producing a first phase difference signal, and a PLL circuit having an oscillator for generating the clock signal based on the first phase difference signal, the adjusting means comprises a phase shifter for shifting a phase of a wobble signal from the frequency divider in accordance with the phase adjusting signal, and a second PLL for producing a recording clock signal in synchronism with a phase of the wobble signal frequency signal.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.